1. Field of the Invention
The invention concerns a method for calibration of a magnetic resonance acquisition channel possessing a magnetic resonance acquisition antenna in a magnetic resonance system. Moreover, the invention concerns a calibration data determination device for a magnetic resonance system, as well as a magnetic resonance system.
2. Description of the Prior Art
Magnetic resonance tomography has become a widespread method for the acquisition of images of the inside of a body. In this method, the body to be examined is exposed to a relatively strong basic magnetic field, for example of 1.5 Tesla or, in newer systems (known as “high magnetic field systems”), of 3 Tesla or even more (presently 7 Tesla and 11 Tesla). A radio-frequency excitation signal (known as the B1 field) is then emitted with a suitable antenna device, which causes the nuclear spins of specific atoms excited to resonance by this radio-frequency field to be tilted by a specific flip angle relative to the magnetic field lines of the basic magnetic field. The radio-frequency signal (known as the magnetic resonance signal) radiated upon relaxation of the nuclear spins is then detected with suitable antenna arrangements (called “magnetic resonance antenna arrangements” in the following). The raw data so acquired are used to reconstruct the desired image data. For spatial coding, defined magnetic field gradients are superimposed on the basic magnetic field during the transmission and the readout or acquisition of the radio-frequency signals.
Today images with high signal-to-noise ratio are normally acquired with antennas in the form known as local coils. The local coils can be executed as loop antennas or as butterfly antennas, for example. Stripline antennas are also used in high field systems. Generally, multiple local coils are used in parallel that individually supply the signal acquired by them to the acquisition electronics via separate acquisition channels.
FIG. 1 schematically shows a design for two such parallel acquisition channels 10. The voltage signal induced in the coil 11 by a magnetic resonance signal is amplified with a low-noise preamplifier 12 (generally designated as an LNA) and is finally relayed to the acquisition electronics 16 via cables 13, 15. High field systems are used to improve the signal-to-noise ratio, even in high-resolution images. Their basic field strengths are presently 3 Tesla or more. Theoretically, a quadrupling of the received power (i.e. an increase by 6 database) results with a doubling of the basic field strength. While maximum signal powers of only −27 dBm typically occur at the input of the preamplifier at 1.5 Tesla, these are already typically −21 dBm at maximum at 3 T. The preamplifier must operate with nearly no distortion in the entire range of the powers, i.e. from thermal noise up to the maximum MR signal. This is still possible only to a limited extent in the high maximum powers that occur in high field systems. Therefore, often at least one additional, switchable amplifier 14 (most often an RCCS SGA=Receive Coil Channel Selector Switchable Gain Amplifier) operates in the further acquisition chain to mitigate the dynamic requirements, which additional amplifier 14 at the same time also forms a switching arrangement to switch over the antenna to different inputs of the acquisition electronics 16. The RCCS SGA 14 and the acquisition electronics 16 are thus respectively fashioned as joint apparatuses for multiple parallel acquisition channels 10, which is different than is shown in FIG. 1. For signals with lower maximum power, the amplifier 14 is switched by a switching signal S so that the amplifier additionally amplifies. Given very strong signals, the amplifier 14 is switched so that only a small amplification or no additional amplification occurs. The noise factor of the acquisition chain in the small signal case is therefore normally much better than in the large signal case because a higher amplification minimizes the contribution of what is known as the “backend” (the cable 15 and the acquisition electronics 16) to the total noise factor.
Switchable amplifiers 14 have previously been connected in the acquisition chain only after the LNA 14 since a switchable amplifier is possible only at the expense of poorer noise adaptation (increased LNA noise). In order to be able to precisely determine the absolute gain of the RCCS SGA 14 for both states (high gain and low gain), a calibration of the crossover switch must occur. For this the amplification of the RCCS SGA 14 is presently measured via a test signal in that a signal with defined signal level is fed in at the input of the RCCS SGA 14. This occurs hardware outlay due to the need to switch the test signals to all possible input paths of the RCCS SGA 14. For a theoretically possible amplification switch-over at the input of the first preamplifier 12 of the acquisition chain (indicated as an option by the dashed-line arrows in FIG. 1), the test signal would have to be transferred through the entire acquisition chain, starting from the output of the local coil 11 itself. This would be connected with a very high wiring and switching cost.